1. Field of the Invention
The present invention relates to a vacuum-insulated, double-walled metal- structure, such as, for example, vacuum bottles, nonfreezing pipes for example, and a method for its production.
2. Description of the Prior Art
A vacuum-insulated, double-walled metal structure, for example, a vacuum bottle, generally comprises inner and outer metal shells seamed together at their opening ends, a space formed between the inner and outer shells being exhausted of air. In order to increase the thermal insulating properties of the vacuum bottle, it is common practice to coat the surfaces of the inner and outer shells defining the vacuum space with a reflective layer which prevents any radiation from the inside of the bottle to the outside, and vice versa. Such a reflective layer is generally made of a metal such as silver, nickel or copper, deposited on the surfaces of the inner and outer shells defining the vacuum space by electroplating or electroless plating. However, the plating is time-consuming, resulting in increase in a production cost of the vacuum bottle.
To solve such a problem, it has been proposed to cover the outer surface of the inner shell with an aluminum or copper foil which serves as a reflective layer, in laid open Japanese patent No. 61-31111. The use of aluminum or copper foil makes it possible to reduce the time required for the formation of the reflective layer on the inner shell, but it also has the following disadvantages as well as those of the prior art.
In producing a vacuum bottle, it is necessary to heat the double-walled bottle during exhausting air from the space between the inner and outer shells to remove absorbed gases from the surfaces of inner and outer shells as well as to clean the surfaces of the inner and outer shells defining the vacuum space. However, if the heat treatment is carried out under the presence of air, the aluminum or copper foil is oxidized as well as the plating, resulting in considerable decrease in the reflective properties.
In order to prevent the reflective layer from oxidizing, the heat or degassing treatment has been generally performed after exhausting air from the space between the inner and outer shells so that the pressure in the space is reduced to a high vacuum of not less than 1.times.10.sup.-3 Torr. However, the temperature rise of the inner shell is considerably decreased since the thermal transfer due to convection scarcely occur under such a high vacuum. Thus, the heat treatment takes a lot of time. In addition, it is difficult to remove the absorbed gases from the outer surface of the inner shell sufficiently. If the gases adhered to or absorbed in the surfaces of inner and outer shells are not removed sufficiently therefrom during exhaustion, they would gradually leave the surfaces of the inner and outer shells afterward, reducing considerably the vacuum in the space, and so decreasing the thermal insulating properties of the vacuum bottle.
In order to prevent the vacuum bottle from lowering of the thermal insulating properties, there have widely been used getters which absorb gases released from the surfaces of the inner and outer shells. However, such a getter is generally of a pellet or disk form, so that it is required to use a fixture or a suitable means for fixing it to the inner or outer shell. Thus, the use of such a fixture is troublesome and causes increase in the production cost of the vacuum bottle.